PROJECT SUMMARY/ABSTRACT While it is established that the incidence of gastric ulcers in the elderly is primarily due to increased use of NSAIDs and other medications, and increased H. pylori infection, the tissue's ability to repair gastric injuries is impaired. Our impression that ulcers heal normally in the aged stomach is false. The available treatments for peptic ulcer are essentially based on gastric acid suppression with anti-secretory drugs and the eradication of H. pylori infection. Despite the evidence that shows ulcers and their complications are a burden that is focused in the elderly, there is a lack of knowledge as to the molecular mechanisms that disrupt repair even after H. pylori eradication within the aged stomach. The objectives of this proposal are to 1) develop an understanding for the process of gastric regeneration, and to then 2) identify the mechanism by which the basic aging process of the stomach leads to an epithelium incapable of repair in response to injury. The acquisition of such knowl- edge is the first step in a continuum of research required to achieve our long-term goal to understand the proc- ess of gastric epithelial repair in response to injury. The central hypothesis is that loss of Hedgehog signaling within the normal stem cell environment of the aged stomach results in impaired regeneration of the stomach in response to injury. The hypothesis has been formulated on the basis of preliminary data that show during re- pair of the stomach in response to ulceration there is the emergence of Spasmolytic Polypeptide/TFF2- Expressing Metaplasia (SPEM) cells at the base of ulcer margin. Our data suggest that SPEM represents the major reparative lineage responsible for wound healing after severe gastric ulcer injury. We have also identi- fied that the cell surface protein CD44, in particular its variant isoforms (CD44v), marks a population of cells within reparative SPEM glands that express Hedgehog receptor Patched (Ptch). Moreover, Hedgehog proteins Sonic Hedgehog (Shh) and Indian Hedgehog (Ihh) are key regulatory factors that play reparative roles in re- sponse to injury by regulating macrophage infiltration and CD44v expression, respectively. Guided by strong preliminary data, this hypothesis is tested by pursuing two specific aims: 1) what is the mechanism by which Hedgehog signaling regulates gastric epithelial repair? And, 2) what is the mechanism by which the aging process of the stomach leads to disrupted repair? The hypothesis will be tested using bone marrow chimeras, heterochronic and isochronic parabioses, a mouse injury/transplantation model using mouse- and human- derived gastric organoids and lineage mapping experiments. At the completion of these studies, we expect to define the molecular events that are crucial for the repair of the gastric epithelium. Outcomes from the pro- posed studies are expected to have significantly advanced our current knowledge of the mechanism by which aging leads to disrupted epithelial repair. The rationale that underlies the research proposed is to develop the potential for testing therapeutic target efficacy and other strategies, such as stem-cell therapy, for the regen- eration of the aged stomach.